I believe that the most important thing to do in order to get started on designing a game of any sort is to write out a complete game design document. Something that explains the mechanics, story boards (if applicable), level progression, character progressions and general flow of events. This in itself will then give you the questions to ask and provide a guideline of the solutions you will require to achieve your goal. Your game design document should be fairly large (spanning at least a few pages even for the simplest of ideas.) You want to focus on defining what the game is completely, imagine that your game design document is being given to someone that know's what a video game is but has never played a platformer, side scroller or 2.5D game.
The key notes you want to address in the game design document are "what platform am I targeting " "what is the game about?" "how does one play the game?" "how does the game start?" "what can the player do?" "what does the player do throughout the middle of the game?" "how does the game end?" "why does the character do what he does in the game?" "how do controls work?". Expand on these with as much detail as possible. The point behind all of this is that it will start letting you address individual aspects of the design in such a way that you kind of set up a road map all on your own as it relates to you'r project.
Some examples, "What platform am I targeting?" this in itself now gives you the question of "What engine or libraries do I use? What hardware API is available (Direct X or Open GL) and what should I learn next?" "How does one play the game?" Will get you to start thinking about how to build your framework to support aspects of your game. In the idea of a 2.5D sidescroller / platformer you already know that you will be looking at basic collision, basic dual axis (2d) movement systems, running and jumping support. If it turns out that there will be some beat em' up aspects of the game you know you will need to add in support for combat / attack animations, health, strength and defenses and so on. As you continue down this road you will start getting to more and more specific questions, questions that later can be answered using a Technical Design Document. As you go through the process of writing a technical design document you will further narrow down exactly what it is you need and what order you will work on these things for.
In short there is no standard work flow or road map to game development, each game requires it's own unique technologies and functionality. Granted most games follow a similar path where you start at the bottom building framework that could support the methods and properties you would need to get the game up and running but how you get those going and where you put them is very dependent on what you are doing. Your game design document and technical design document turn in to your guide on where to go.